JP2017198660A - Gas detector and gas detection method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas detector which can detect a hydrogen-containing gas stably and sensitively and has an excellent power saving property.SOLUTION: The gas detector includes: a measurement circuit having a gas sensor and a measurement unit; and a determination circuit. Each of a plurality of detection cells in the gas sensor includes: a first electrode; a second electrode having an exposed surface exposed from an insulating film; and a metal oxide layer located between the first and second electrodes. The resistance values of the detection cells drop when a gas containing hydrogen atoms becomes in contact with the second electrode. The measurement circuit monitors resistance values of the detection cells. The determination circuit determines whether the gas containing hydrogen atoms has been detected, based on the change in at least one of the resistance values.SELECTED DRAWING: Figure 14

Description

  The present disclosure relates to a gas detection device including a gas sensor.

Patent Document 1 discloses a gas sensor that detects the presence of hydrogen gas as a change in resistance value. This gas sensor includes a material in which palladium (Pd) and glass are added to tantalum pentoxide (Ta ₂ O ₅ ), and a platinum (Pt) electrode sandwiching the material.

Non-Patent Document 1 discloses a Pt / Ta ₂ O ₅ Schottky diode for hydrogen sensing. In this Schottky diode, hydrogen molecules dissociate into hydrogen atoms on the surface of the catalyst Pt.

JP 59-58348 A

Sensors and Actuators A 172 (2011) 9-14

  The present disclosure provides a gas detection device that can detect a hydrogen-containing gas stably with high sensitivity and is excellent in power saving.

  A gas detection device according to an aspect of the present disclosure includes a gas sensor including a plurality of detection cells covered with an insulating film, and at least one measuring device that monitors a plurality of resistance values of the plurality of detection cells. A circuit and a determination circuit that determines whether or not a gas containing hydrogen atoms has been detected based on a change in at least one of the plurality of resistance values. Each of the plurality of detection cells is disposed between a first electrode, a second electrode having an exposed surface exposed from the insulating film, and the bulk region. And a metal oxide layer including a local region surrounded by the bulk region and having a larger oxygen deficiency than the bulk region. The resistance value of the detection cell decreases when the gas contacts the second electrode.

  The gas detection device according to one embodiment of the present disclosure is excellent in power saving and can stably detect a hydrogen-containing gas with high sensitivity.

FIG. 1A is a cross-sectional view illustrating an example of a gas detection element according to the first embodiment. FIG. 1B is a top view illustrating an example of the gas detection element according to the first embodiment. FIG. 2A is a cross-sectional view illustrating an example of the method for manufacturing the gas detection element according to the first embodiment. FIG. 2B is a cross-sectional view illustrating an example of the method for manufacturing the gas detection element according to the first embodiment. FIG. 2C is a cross-sectional view illustrating an example of the method for manufacturing the gas detection element according to the first embodiment. FIG. 2D is a cross-sectional view illustrating an example of the method for manufacturing the gas detection element according to the first embodiment. FIG. 2E is a cross-sectional view illustrating an example of the method for manufacturing the gas detection element according to the first embodiment. FIG. 2F is a cross-sectional view illustrating an example of the method for manufacturing the gas detection element according to the first embodiment. FIG. 2G is a cross-sectional view illustrating an example of the method for manufacturing the gas detection element according to the first embodiment. FIG. 3 is a diagram illustrating an example of state transition of the gas detection element according to the first embodiment. FIG. 4 is a cross-sectional view showing a gas detection element according to a modification of the first embodiment. FIG. 5 is a diagram showing an evaluation system for a gas detection element according to a modification of the first embodiment. FIG. 6 is a diagram illustrating an evaluation result of the gas detection element according to the modified example of the basic configuration. FIG. 7 is a diagram illustrating a hydrogen detection time distribution in a plurality of evaluation results of the gas detection element according to the modified example of the basic configuration. FIG. 8 is a cross-sectional view illustrating an example of a gas sensor according to the first embodiment. FIG. 9 is a circuit diagram illustrating an example of a gas detection circuit according to the first embodiment. FIG. 10A is a diagram illustrating variation for each test of the hydrogen detection time of the gas detection element according to the first embodiment. FIG. 10B is a diagram illustrating variation for each test of the hydrogen detection time of the gas sensor according to the first embodiment. FIG. 11 is a circuit diagram illustrating an example of a gas detection circuit according to the second embodiment. FIG. 12 is a circuit diagram illustrating an example of a gas detection circuit according to the third embodiment. FIG. 13 is a circuit diagram illustrating an example of a gas detection circuit according to the third embodiment. FIG. 14 is a schematic diagram illustrating an example of a gas detection circuit according to the first, second, and third embodiments.

(Knowledge that became the basis of this disclosure)
As a result of intensive studies by the present inventors, it has been found that the conventional gas sensor has the following problems.

  In the conventional gas sensor, the element for detecting the gas is heated to 100 ° C. or higher in order to improve the sensitivity for detecting the hydrogen-containing gas. Therefore, the power consumption of the conventional gas sensor is about 100 mW even at the minimum. Therefore, when the gas sensor is always used in an ON state, there is a problem that power consumption increases.

  The gas detection device according to one embodiment of the present disclosure can detect a hydrogen-containing gas with high sensitivity and stability, and is excellent in power saving.

  Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

  In the drawings, elements representing substantially the same configuration, operation, and effect are denoted by the same reference numerals, and description thereof is omitted. In addition, numerical values, materials, compositions, shapes, film formation methods, connection relationships between components, and the like described below are all merely examples for specifically describing embodiments of the present disclosure, and the present disclosure It is not limited to these. In addition, among the constituent elements in the following embodiments, constituent elements that are not described in the independent claims indicating the highest concept are described as optional constituent elements.

(First embodiment)
The gas sensor according to the first embodiment includes a plurality of gas detection elements. The plurality of gas detection elements have substantially the same structure and substantially the same size. Each of the plurality of gas detection elements has a metal-insulating film-metal (MIM) structure in which a resistance film (metal oxide layer) and a metal film are stacked. Each gas detection element can detect a hydrogen-containing gas without being heated by a heater by utilizing self-heating and gas sensitivity in a local region formed in the resistance film. Here, the hydrogen-containing gas is a general term for gases composed of molecules having hydrogen atoms, and as an example, may include hydrogen, methane, alcohol, and the like.

  Below, the structure, manufacturing method, and characteristic of one gas detection element are demonstrated first, and the gas sensor comprised using a several gas detection element is demonstrated after that.

[Configuration of gas detection element]
FIG. 1A is a cross-sectional view illustrating a configuration example of the gas detection element 100 according to the first embodiment.

  FIG. 1B is a top view illustrating a configuration example of the gas detection element 100 according to the first embodiment. The cross section in FIG. 1A corresponds to the cross section seen in the direction of the arrow along the cutting line 1A-1A in FIG. 1B.

  The gas detection element 100 includes a substrate 101, an insulating film 102 formed on the substrate 101, a first electrode 103, a second electrode 106, a first electrode 103, and a second electrode formed above the insulating film 102. A resistance film 104, an insulating film 107, a via 108, and a wiring 109 sandwiched between the electrodes 106 are provided. The main surface of the first electrode 103 and the main surface of the second electrode 106 are arranged to face each other, and the resistance film 104 is in contact with the main surface of the first electrode 103 and the main surface of the second electrode 106. Has been placed.

  The insulating film 107 is provided with an opening 107a for bringing the second electrode 106 into contact with a gas to be inspected. In other words, the insulating film 107 covers the first electrode 103, the second electrode 106, and the resistance film 104, and at least a part of the upper surface of the second electrode 106 (the other surface facing the main surface described above) The insulating film 107 is exposed without being covered.

  The resistance film 104 is interposed between the first electrode 103 and the second electrode 106. The resistance value of the resistance film 104 reversibly changes based on an electrical signal applied between the first electrode 103 and the second electrode 106. For example, the resistance state of the resistance film 104 reversibly transits between a high resistance state and a low resistance state according to a voltage (potential difference) applied between the first electrode 103 and the second electrode 106. In addition, the resistance state of the resistance film 104 changes from, for example, a high resistance state to a low resistance state in accordance with the hydrogen-containing gas in contact with the second electrode 106.

  Inside the resistance film 104, a local region 105 that is disposed in contact with the second electrode 106 and is not in contact with the first electrode 103 is provided. The degree of oxygen deficiency in the local region 105 is larger than the oxygen deficiency around it (that is, the bulk region of the resistance film 104). The degree of oxygen deficiency in the local region 105 depends on the application of an electrical signal between the first electrode 103 and the second electrode 106 and the presence or absence of a hydrogen-containing gas in the gas in contact with the second electrode 106. It changes reversibly. The local region 105 is a minute region including a filament (conductive path) composed of oxygen defect sites.

  In the insulating film 107, the via 108 penetrates the insulating film 107 and is connected to the second electrode 106 in a portion covering the upper surface of the second electrode 106. A wiring 109 is disposed on the via 108.

  Note that in this disclosure, the “oxygen deficiency” of a metal oxide refers to the amount of oxygen in the metal oxide relative to the amount of oxygen in the stoichiometric composition composed of the same elements as the metal oxide. The ratio of the deficiency amount (here, the deficiency amount of oxygen is a value obtained by subtracting the amount of oxygen in the metal oxide from the amount of oxygen in the stoichiometric metal oxide). If there are multiple stoichiometric metal oxides composed of the same elements as the metal oxide, the oxygen deficiency of the metal oxide is determined by the metal oxide having the stoichiometric composition. Is defined based on the one having the highest resistance value. A metal oxide having a stoichiometric composition is more stable and has a higher resistance value than a metal oxide having another composition.

For example, when the metal is tantalum (Ta), the oxide having the stoichiometric composition according to the above definition is Ta ₂ O ₅ , and can be expressed as TaO _2.5 . The oxygen deficiency of TaO _2.5 is 0%, and the oxygen deficiency of TaO _1.5 is oxygen deficiency = (2.5−1.5) /2.5=40%. In addition, the oxygen excess metal oxide has a negative oxygen deficiency. In the present disclosure, the oxygen deficiency may take a positive value, 0, or a negative value unless otherwise specified.

  An oxide having a low degree of oxygen deficiency has a high resistance value because it is closer to an oxide having a stoichiometric composition, and an oxide having a high degree of oxygen deficiency has a low resistance value because it is closer to the metal constituting the oxide.

“Oxygen content” is the ratio of oxygen atoms to the total number of atoms. For example, the oxygen content of Ta ₂ O ₅ is the ratio of oxygen atoms to the total number of atoms (O / (Ta + O)), which is 71.4 atm%. Therefore, the oxygen-deficient tantalum oxide has an oxygen content greater than 0 and less than 71.4 atm%.

  The local region 105 is formed in the resistance film 104 by applying an initial break voltage between the first electrode 103 and the second electrode 106. In other words, the initial break voltage is a voltage applied between the first electrode 103 and the second electrode 106 in order to form the local region 105. The initial break voltage may be a voltage whose absolute value is larger than the write voltage. The write voltage is a voltage applied between the first electrode 103 and the second electrode 106 in order to reversibly transition the resistance film 104 between a high resistance state and a low resistance state. Alternatively, the initial break voltage may be a voltage whose absolute value is smaller than the write voltage. In this case, the initial break voltage may be repeatedly applied or continuously applied for a predetermined time. By applying the initial break voltage, a local region 105 that is in contact with the second electrode 106 and not in contact with the first electrode 103 is formed as shown in FIG. 1A.

  The local region 105 is considered to include a filament (conductive path) composed of oxygen defect sites. The size of the local region 105 is a minute size suitable for a filament necessary for flowing current. Filament formation in the local region 105 is described using a percolation model.

  The percolation model is based on the theory that assuming a random distribution of oxygen defect sites in the local region 105 and the density of oxygen defect sites exceeds a certain threshold, the probability that the connection of oxygen defect sites increases. It is a model.

  According to the percolation model, the filament is formed by connecting a plurality of oxygen defect sites in the local region 105, and the resistance change in the resistance film 104 is expressed through the generation and disappearance of the oxygen defect sites in the local region 105.

  Here, the “oxygen defect” means that oxygen is missing from the stoichiometric composition in the metal oxide, and the “density of oxygen defect site” also corresponds to the degree of oxygen deficiency. That is, as the oxygen deficiency increases, the density of oxygen defect sites also increases.

  Only one local region 105 may be formed in one resistive film 104 of the gas detection element 100. The number of local regions 105 formed in the resistance film 104 can be confirmed by, for example, EBAC (Electron Beam Absorbed Current) analysis.

  When the local region 105 exists in the resistance film 104, the current in the resistance film 104 flows intensively in the local region 105 when a voltage is applied between the first electrode 103 and the second electrode 106.

  The size of the local region 105 is small. Therefore, the local region 105 generates heat due to, for example, a current of about several tens of μA flowing when reading the resistance value, and this heat generation causes a considerable temperature increase. When a current of about several tens of μA flows, the power consumption is less than 0.1 mW.

  Therefore, the second electrode 106 is made of a catalytic metal (eg, Pt), and the local region 105 is in contact with the second electrode 106. According to these configurations, the second electrode 106 is heated by heat generation in the local region 105, and hydrogen atoms are efficiently dissociated from the hydrogen-containing gas.

  When there is a hydrogen-containing gas in the gas to be inspected, hydrogen atoms are dissociated from the hydrogen-containing gas in the second electrode 106, and the dissociated hydrogen atoms are combined with oxygen atoms in the local region 105, and as a result. The resistance value of the local region 105 decreases.

  Thus, the gas detection element 100 has a characteristic that the resistance value between the first electrode 103 and the second electrode 106 decreases when the second electrode 106 contacts the hydrogen-containing gas. According to the characteristics, when the gas to be inspected is in contact with the second electrode 106, hydrogen contained in the gas is detected by detecting a decrease in the resistance value between the first electrode 103 and the second electrode 106. The contained gas can be detected.

  Note that, regardless of whether the local region 105 is in a high-resistance state or a low-resistance state, the resistance value is reduced when the hydrogen-containing gas contacts the second electrode 106. Therefore, the hydrogen-containing gas can be detected by the gas detection element 100 in which the local region 105 is in either the high resistance state or the low resistance state. However, the gas detection element 100 in which the local region 105 is electrically set in advance in a high resistance state may be used so that the decrease in the resistance value can be detected more clearly.

  Below, the detail of the gas detection element 100 for obtaining the stable resistance change characteristic is demonstrated.

  The resistance film 104 is made of an oxygen-deficient metal oxide. The base metal of the metal oxide is tantalum (Ta), hafnium (Hf), titanium (Ti), zirconium (Zr), niobium (Nb), tungsten (W), nickel (Ni), iron (Fe), etc. At least one of these transition metals and aluminum (Al) may be selected. Since transition metals can take a plurality of oxidation states, different resistance states can be realized by oxidation-reduction reactions.

  Here, the oxygen-deficient metal oxide is a metal oxide having a greater degree of oxygen deficiency than a metal oxide having the same metal element and having a stoichiometric composition. Stoichiometric metal oxides are typically insulators, whereas oxygen-deficient metal oxides typically have semiconducting properties. By using the oxygen-deficient metal oxide for the resistance film 104, the gas detection element 100 can realize a resistance change operation with good reproducibility and stability.

For example, when hafnium oxide is used as the metal oxide constituting the resistance film 104, when the composition is expressed as HfO _x and x is 1.6 or more, the resistance value of the resistance film 104 changes stably. Can be made. In this case, the film thickness of the hafnium oxide may be 3 to 4 nm.

Further, when zirconium oxide is used as the metal oxide constituting the resistance film 104, when the composition is expressed as ZrO _x , when x is 1.4 or more, the resistance value of the resistance film 104 changes stably. Can be made. In this case, the film thickness of the zirconium oxide may be 1 to 5 nm.

Further, when tantalum oxide is used as the metal oxide constituting the resistance film 104, when the composition is expressed as TaO _x and x is 2.1 or more, the resistance value of the resistance film 104 changes stably. Can be made.

  The composition of each metal oxide layer can be measured using Rutherford backscattering method.

  As materials of the first electrode 103 and the second electrode 106, for example, Pt (platinum), Ir (iridium), Pd (palladium), Ag (silver), Ni (nickel), W (tungsten), Cu ( It is selected from copper, Al (aluminum), Ta (tantalum), Ti (titanium), TiN (titanium nitride), TaN (tantalum nitride) and TiAlN (titanium nitride aluminum).

  Specifically, the second electrode 106 is a gas molecule containing a hydrogen atom, such as platinum (Pt), iridium (Ir), palladium (Pd), or an alloy containing at least one of them. From a material having a catalytic action to dissociate hydrogen atoms from The first electrode 103 is formed of a metal such as tungsten (W), nickel (Ni), tantalum (Ta), titanium (Ti), aluminum (Al), tantalum nitride (TaN), or titanium nitride (TiN). The standard electrode potential may be made of a material lower than that of the metal constituting the oxide. The standard electrode potential represents a characteristic that the higher the value is, the more difficult it is to oxidize.

  As the substrate 101, for example, a silicon single crystal substrate or a semiconductor substrate can be used, but the substrate 101 is not limited thereto. Since the resistance film 104 can be formed at a relatively low substrate temperature, for example, the resistance film 104 can be formed on a resin material or the like.

  The gas detection element 100 may further include, for example, a fixed resistor, a transistor, or a diode as a load element electrically connected to the resistance film 104.

[Manufacturing method and operation of gas detection element]
Next, an example of a method for manufacturing the gas detection element 100 will be described with reference to FIGS. 2A to 2G.

  First, as shown in FIG. 2A, an insulating film 102 having a thickness of 200 nm is formed on a substrate 101 made of, for example, single crystal silicon by a thermal oxidation method. Then, for example, a Pt thin film with a thickness of 100 nm is formed on the insulating film 102 as the first electrode 103 by a sputtering method. Note that an adhesion layer of Ti, TiN, or the like can be formed between the first electrode 103 and the insulating film 102 by a sputtering method. Thereafter, an oxygen-deficient metal oxide layer to be the resistance film 104 is formed on the first electrode 103 by, for example, reactive sputtering using a Ta target. Thus, the resistance film 104 is formed.

  Here, with respect to the thickness of the resistance film 104, there is a disadvantage that the initial resistance value becomes too high if it is too thick, and there is a disadvantage that a stable resistance change cannot be obtained if it is too thin. For the above reasons, it may be about 1 nm or more and 8 nm or less.

  Next, a Pt thin film having a thickness of, for example, 150 nm is formed as the second electrode 106 on the resistance film 104 by a sputtering method.

  Next, as shown in FIG. 2B, a photoresist mask 300 is formed by a photolithography process. After that, as shown in FIG. 2C, the first electrode 103, the resistance film 104, and the second electrode 106 are formed in the shape of an element by dry etching using a mask 300.

  After that, as illustrated in FIG. 2D, an insulating film 107 is formed so as to cover the insulating film 102, the first electrode 103, the resistance film 104, and the second electrode 106. Then, a via hole 107 b reaching a part of the upper surface of the second electrode 106 is provided in the insulating film 107 by etching.

  Next, as shown in FIG. 2E, a conductor film 108 'is formed so as to fill the upper surface of the insulating film 107 and the inside of the via hole 107b. Thereafter, as shown in FIG. 2F, the conductor film 108 'on the insulating film 107 is removed by CMP (Chemical Mechanical Polishing) to form the via 108 in the via hole 107b. Further, a new conductor film is disposed on the insulating film 107 and patterned to form a wiring 109 connected to the via 108.

  Next, as shown in FIG. 2G, an opening 107a through which a part of the upper surface of the second electrode 106 is exposed is provided in the insulating film 107 by etching.

  Thereafter, by applying an initial break voltage between the first electrode 103 and the second electrode 106, the local region 105 shown in FIG. 1A is formed in the resistance film 104, and the gas detection element 100 is completed.

  Here, the resistance change characteristic by voltage application of the gas detection element 100 is demonstrated based on the measurement result by a sample element. In addition, the resistance change characteristic by the hydrogen-containing gas of the gas detection element 100 will be described later.

  FIG. 3 is a graph showing resistance change characteristics actually measured with the sample elements.

In the gas detection element 100 which is a sample element from which the measurement result of FIG. 3 is obtained, the size of the first electrode 103, the second electrode 106, and the resistance film 104 is 0.5 μm × 0.5 μm (area 0.25 μm). ² ). Further, when the composition of the tantalum oxide as the resistance film 104 is expressed as TaO _y , y = 2.47. Furthermore, the thickness of the resistance film 104 is 5 nm. When a reading voltage (for example, 0.4 V) is applied between the first electrode 103 and the second electrode 106 for such a gas detection element 100, the initial resistance value RI is about 10 ⁷ to 10. ⁸ Ω.

  As shown in FIG. 3, when the resistance value of the gas detection element 100 is the initial resistance value RI (a value higher than the resistance value HR in the high resistance state), the initial break voltage is set to the first electrode 103 and the second electrode. The resistance state changes when applied between the first and second electrodes 106. Thereafter, between the first electrode 103 and the second electrode 106 of the gas detection element 100, for example, two kinds of voltage pulses having a pulse width of 100 ns and different polarities (a positive voltage pulse and a negative voltage) are used as writing voltages. When the pulses are applied alternately, the resistance value between the first electrode 103 and the second electrode 106 changes as shown in FIG.

  That is, when a positive voltage pulse (pulse width 100 ns) is applied between the electrodes as a writing voltage, the resistance value between the first electrode 103 and the second electrode 106 changes from the low resistance value LR to the high resistance value HR. To increase. On the other hand, when a negative voltage pulse (pulse width 100 ns) is applied between the electrodes as a writing voltage, the resistance value between the first electrode 103 and the second electrode 106 changes from the high resistance value HR to the low resistance value LR. Decrease. Note that the polarity of the voltage pulse is “positive” when the potential of the second electrode 106 is high with reference to the potential of the first electrode 103, and the second electrode 106 with reference to the potential of the first electrode 103. The case where the potential is low is “negative”.

  By utilizing such a resistance change characteristic due to voltage application, by applying a positive voltage pulse between the first electrode 103 and the second electrode 106 before starting monitoring of the hydrogen-containing gas, The hydrogen-containing gas can be detected using the gas detection element 100 set to the high resistance state (HR). Thereby, since the fall of resistance value can be detected more clearly compared with the case where hydrogen content gas is detected using gas detection element 100 of a low resistance state (LR), the detection characteristic of hydrogen content gas improves.

[Modification of gas detection element]
FIG. 4 is a cross-sectional view illustrating a configuration example of a gas detection element according to a modification of the first embodiment. Hereinafter, only different points from the gas detection element 100 of the first embodiment will be described.

  In the gas detection element 200 of this modification, the resistance film 204 has two layers of a first metal oxide layer 204 a in contact with the first electrode 103 and a second metal oxide layer 204 b in contact with the second electrode 106. It differs from the gas detection element 100 of 1st Embodiment by the point comprised by laminating | stacking. Note that the resistance film 204 is not limited to two layers, and three or more metal oxide layers may be stacked.

  In the first metal oxide layer 204a and the second metal oxide layer 204b, a local region 105 in which the degree of oxygen deficiency reversibly changes in accordance with application of an electric pulse and a hydrogen-containing gas is provided. The local region 105 is formed in contact with the second electrode 106 through at least the second metal oxide layer 204b.

  In other words, the resistance film 204 includes a stacked structure of at least a first metal oxide layer 204a including a first metal oxide and a second metal oxide layer 204b including a second metal oxide. The first metal oxide layer 204a is disposed between the first electrode 103 and the second metal oxide layer 204b, and the second metal oxide layer 204b is the first metal oxide layer. It is arranged between 204 a and the second electrode 106.

  The thickness of the second metal oxide layer 204b may be smaller than the thickness of the first metal oxide layer 204a. In this case, a structure in which the local region 105 is not in contact with the first electrode 103 can be easily formed. The oxygen deficiency of the second metal oxide layer 204b may be smaller than the oxygen deficiency of the first metal oxide layer 204a. In this case, since the resistance value of the second metal oxide layer 204b is higher than the resistance value of the first metal oxide layer 204a, most of the voltage applied to the resistance film 204 is almost equal to the second metal oxide layer 204b. To be applied. This configuration is useful for, for example, concentrating the initial break voltage on the second metal oxide layer 204 b and reducing the initial break voltage necessary for forming the local region 105.

  Further, in the present disclosure, when the metal constituting the first metal oxide layer 204a and the second metal oxide layer 204b is the same, the term “oxygen content” is used instead of “oxygen deficiency”. May be used. “High oxygen content” corresponds to “low oxygen deficiency” and “low oxygen content” corresponds to “high oxygen deficiency”.

  However, as will be described later, the resistance film 204 according to the present embodiment is not limited to the case where the metals constituting the first metal oxide layer 204a and the second metal oxide layer 204b are the same. Alternatively, different metals may be used. That is, the first metal oxide layer 204a and the second metal oxide layer 204b may be different metal oxides.

  When the first metal constituting the first metal oxide layer 204a and the second metal constituting the second metal oxide layer 204b are the same, the oxygen content corresponds to the degree of oxygen deficiency. is there. That is, when the oxygen content of the second metal oxide is greater than the oxygen content of the first metal oxide, the oxygen deficiency of the second metal oxide is less than the oxygen deficiency of the first metal oxide. .

  The resistance film 204 includes a local region 105 in the vicinity of the interface between the first metal oxide layer 204a and the second metal oxide layer 204b. The oxygen deficiency in the local region 105 is larger than the oxygen deficiency in the second metal oxide layer 204b and is different from the oxygen deficiency in the first metal oxide layer 204a.

  By applying an initial break voltage between the first electrode 103 and the second electrode 106, the local region 105 is formed in the resistance film 204. The initial break voltage is in contact with the second electrode 106, penetrates the second metal oxide layer 204b, partially penetrates the first metal oxide layer 204a, and is not in contact with the first electrode 103. Region 105 is formed.

  An example of evaluation of resistance change characteristics due to the hydrogen-containing gas of the gas detection element 200 configured as described above will be described.

  FIG. 5 is a block diagram illustrating an example of an evaluation system used for evaluating the gas detection element 200. An evaluation system 900 shown in FIG. 5 includes a sealed container 910 that stores the gas detection element 200, a detection power source 920 that generates a detection voltage, and a current measuring device 930. The hermetic container 910 is connected to a hydrogen cylinder 911 and a nitrogen cylinder 912 via introduction valves 913 and 914, respectively, and is configured to be able to discharge internal gas via an exhaust valve 915.

  FIG. 6 is a graph illustrating an evaluation example of the gas detection element 200. The horizontal axis represents time (au), and the vertical axis represents the current value (au) flowing through the gas detection element 200. In the experiment, first, the gas detection element 200 was placed in the nitrogen gas introduced into the sealed container 910, the detection voltage was applied, and current measurement was started, and then hydrogen gas was introduced into the sealed container 910.

  FIG. 6 shows the result at this time, and the horizontal axis shows two periods of nitrogen introduction and hydrogen introduction. After the introduction of hydrogen gas is started, the current value starts to increase, and when the current value reaches a predetermined threshold current, hydrogen gas is detected. The time from the start of hydrogen gas introduction until the current value increases and reaches a predetermined threshold current is represented as a hydrogen detection time t. After hydrogen detection, the current value further increases and saturates.

  It is known that the hydrogen detection time t becomes longer as the hydrogen concentration contained in the gas to be inspected is thinner and shorter as the concentration is higher. That is, the hydrogen detection time t is an index of time sensitivity that directly represents the time required to detect hydrogen gas with a certain hydrogen concentration, and at the same time, the hydrogen concentration of the hydrogen-containing gas that can be detected within a practical time limit. It is also an indicator of concentration sensitivity associated with the lower limit.

  In this evaluation example, the gas detection element 200 in which the local region 105 is set in a high resistance state by applying a predetermined voltage (reset voltage) in advance between the first electrode 103 and the second electrode 106 is used. It was.

  In the monitoring operation of the hydrogen-containing gas, a detection voltage of 0.6 V is applied between the first electrode 103 and the second electrode 106, hydrogen gas is detected, and the current value is saturated. A current of about 20 μA flowed between the electrode 103 and the second electrode 106.

  Therefore, according to the gas detection element 200, it turns out that hydrogen containing gas can be monitored with the very small power consumption of 0.012 mW at most. The voltage of 0.6 V may be constantly applied between the first electrode 103 and the second electrode 106.

  Note that when a detection voltage of 0.4 V was applied between the first electrode 103 and the second electrode 106, the resistance change due to the hydrogen gas did not occur, and the hydrogen gas could not be detected. This is because when the detection voltage of 0.4 V is applied, heat generation in the local region 105 is insufficient to promote the catalytic action of the second electrode 106, and 0 is necessary to detect hydrogen gas. It is thought that it was necessary to apply a detection voltage of .6V. The detection voltage of 0.6 V in this case is a characteristic that the resistance value between the first electrode 103 and the second electrode 106 decreases when the second electrode 106 is in contact with a gas containing gas molecules having hydrogen atoms. It is an example of the detection voltage which activates.

  After the gas detection element 200 detects the hydrogen-containing gas and the current value is saturated, that is, after the gas detection element 200 detects the hydrogen-containing gas and enters a low resistance state, the first electrode 103 and the first electrode 103 The local region 105 can be reset to a high resistance state by applying a predetermined voltage (reset voltage) between the two electrodes 106.

  FIG. 7 shows the hydrogen detection time t (au) when the experiment for detecting the hydrogen-containing gas again with the gas detection element 200 in which the local region 105 is reset to the high resistance state is repeated a predetermined number of times (for example, 50 times). ) Frequency distribution.

  FIG. 7 shows that when the hydrogen detection operation is repeated with the same gas detection element 200, the hydrogen detection time t varies for each hydrogen detection operation according to a certain distribution. Note that even when the hydrogen detection operation is performed simultaneously on the plurality of gas detection elements 200, the hydrogen detection time t varies from one hydrogen detection element to another according to the same distribution.

  From the above results, the inventors infer the mechanism by which the hydrogen detection time t varies in the gas detection element 200 as follows.

  When the hydrogen-containing gas comes into contact with the second electrode 106, hydrogen atoms are dissociated from the hydrogen-containing gas by the catalytic action of the second electrode 106. The dissociated hydrogen atoms diffuse through the second electrode 106 and reach the local region 105 in an attempt to maintain an equilibrium state.

  The hydrogen atom that has reached the local region 105 causes a reduction reaction in the minute local region 105, and oxygen in the local region 105 reacts with the hydrogen atom. Oxygen defects are newly generated in the local region 105, and the degree of oxygen deficiency in the local region 105 increases. When many oxygen defects are generated in the local region 105, filaments formed from the oxygen defects are easily connected, and the resistance value of the local region 105 is reduced. As a result, it is considered that the current flowing between the first electrode 103 and the second electrode 106 increases.

  However, the position where the oxygen defect occurs in the local region 105 is random, and even if the number of oxygen defects is the same, there are cases where the filament is formed at a position where the filament is easily connected and at a position where it is difficult to connect. . In the former case, the current increase occurs early, and in the latter case, the current increase occurs slowly, and as a result, it is estimated that the hydrogen detection time t varies.

  Note that the above-described operation is not limited to the gas detection element 200, and is considered to occur also in the gas detection element 100 whose main structure is substantially the same as that of the gas detection element 200 and other gas detection elements described later. In addition, it is considered that the above-described operation is not limited to hydrogen gas, but can be generated for various hydrogen-containing gases such as methane and alcohol.

  Variation in the hydrogen detection time t hinders the detection sensitivity and stability of the hydrogen-containing gas by the gas detection element. Specifically, the time required to detect a hydrogen-containing gas with a certain hydrogen concentration becomes unstable (deterioration of time sensitivity), and the lower limit of the hydrogen concentration of the hydrogen-containing gas that can be detected within a practical time limit is limited. There is concern about instability (degradation of density sensitivity).

  That is, the single gas detection element 200 generates heat only by a current for detecting the resistance state, and can detect a hydrogen-containing gas without being heated by a separate heater. There is also a problem that the detection sensitivity is not stable.

  Therefore, as a countermeasure to the problem, a plurality of gas detection elements having substantially the same structure and substantially the same size are provided, and the first electrode and the first electrode in the predetermined number of gas detection elements among the plurality of gas detection elements. A gas sensor that detects a hydrogen-containing gas when the resistance value between the two electrodes decreases is proposed.

[Configuration of gas sensor]
FIG. 8 is a cross-sectional view illustrating a configuration example of the gas sensor 1000 according to the first embodiment. In the gas sensor 1000, a plurality of gas detection elements 200 are arranged on the substrate 101. FIG. 8 illustrates a case where five gas detection elements 200 are arranged. The plurality of gas detection elements 200 constituting the gas sensor 1000 are all configured with substantially the same structure and substantially the same size.

  Here, “substantially the same structure” means, for example, the stacking order of the constituent elements and the coincidence of the material components used, and “substantially the same size” means, for example, the coincidence of the design dimensions. In the actually produced gas sensor 1000, there may be some error (for example, an error of about several percent) in material components and shape dimensions.

[Gas Sensor Manufacturing Method]
For manufacturing the gas sensor 1000, basically, the same manufacturing method as the manufacturing method of the gas detection element 100 shown in FIGS. 2A to 2G is used. The details of the process of FIG. 2A are modified so that the resistive film 204 is formed by stacking two layers of the first metal oxide layer 204a and the second metal oxide layer 204b. The gas sensor 1000 including the plurality of gas detection elements 200 shown in FIG. 8 is manufactured by collectively forming the plurality of gas detection elements 200.

[Gas detection circuit]
FIG. 9 is a circuit diagram showing an example of a gas detection circuit 1010 including the gas sensor 1000 according to the first embodiment.

  The gas detection circuit 1010 includes a plurality of series circuits 1013 formed by connecting one of the gas detection elements 200 and a current measuring device 930 in series, and the measurement circuit 1011 formed by connecting the plurality of series circuits 1013 in parallel. And a power supply circuit 1012 having a detection power supply 920. Each current measuring device 930 is a gas connected to the current measuring device 930 when a detection voltage is applied between the first electrode and the second electrode of the plurality of gas detection elements 200 by the detection power source 920. The current flowing through the detection element is measured.

  The gas detection circuit 1010 detects the hydrogen-containing gas when the current increases in the predetermined number of current measuring devices 930 (that is, the resistance value decreases in the predetermined number of gas detection elements 200). The predetermined number may be one or plural.

  More specifically, the second electrodes 106 of the plurality of gas detection elements 200 are connected to each other via the via 108 and the wiring 109 shown in FIG. 8 and connected to the positive potential terminal of the detection power source 920. In addition, each of the first electrodes 103 of the plurality of gas detection elements 200 is connected to one end of a corresponding current measuring device 930 through, for example, a wiring (not shown). The other end of each current measuring device 930 is connected to the negative potential terminal of the detection power source 920. With the above configuration, a detection voltage is applied by the detection power source 920 between the first electrode 103 and the second electrode 106 of each gas detection element 200.

  In the gas detection circuit 1010, in one of a plurality of current measuring devices 930 each connected to a different gas detection element 200, the predetermined threshold current shown in FIG. The time point is set as a determination point for hydrogen detection. That is, the gas detection circuit 1010 determines that hydrogen has been detected when the first one of the plurality of gas detection elements 200 exceeds a predetermined threshold current.

  FIG. 10A is a diagram illustrating the variation of the hydrogen detection time t of the gas detection element 200 for each test. FIG. 10A shows a result of measuring the hydrogen detection time t by the single gas detection element 200 a plurality of times under the same experimental conditions using the evaluation system 900 shown in FIG. 10A, the horizontal axis represents the test number of the gas detection element 200 used in each experiment, and the vertical axis represents the hydrogen detection time t (au). From FIG. 10A, it can be seen that for each test, the variation in the hydrogen detection time t is large, and the hydrogen detection sensitivity is very unstable.

  FIG. 10B is a diagram showing the variation of the hydrogen detection time t of the gas sensor 1000 for each test. FIG. 10B shows the results of measuring the hydrogen detection time t multiple times under the same experimental conditions using the gas detection circuit 1010 including the gas sensor 1000 shown in FIG. The hydrogen detection time t in FIG. 10B represents the shortest time among the hydrogen detection times t of the plurality of gas detection elements 200 constituting the gas sensor 1000. In the gas sensor 1000, both the magnitude and variation of the hydrogen detection time t are small compared to the single gas detection element 200. That is, good hydrogen detection sensitivity can be stably obtained.

  As described above, according to the gas detection circuit 1010 using the gas sensor 1000 including the plurality of gas detection elements 200 having substantially the same structure and the same size, stable hydrogen detection can be performed. Become. In the present embodiment, the experimental results in the case of hydrogen gas have been described. However, the present invention is not limited to hydrogen gas, and the same effect can be obtained with other hydrogen-containing gas (for example, ammonia gas).

[Supplement]
As shown in FIGS. 9 and 14, the gas detection circuit 1010 includes a measurement circuit 1011 including a gas sensor 1000 and a plurality of current measuring devices 930, and a determination circuit 1014. The gas detection circuit 1010 is an example of the “gas detection device” in the present disclosure, and the current measuring device 930 is an example of the “measurement device” in the present disclosure.

  As shown in FIG. 8, the gas sensor 1000 includes a plurality of gas detection elements 200 covered with an insulating film 107. The gas detection element 200 is an example of the “detection cell” in the present disclosure.

  Each of the gas detection elements 200 includes a first electrode 103, a resistance film 204 disposed on the first electrode 103, and a second electrode 106 disposed on the resistance film 204. The resistance film 204 is an example of the “metal oxide layer” of the present disclosure. The resistance film 204 includes a first metal oxide layer 204a and a second metal oxide layer 204b. The resistance film 204 includes a local region 105 and a bulk region surrounding the local region 105. Here, “surrounding the local region 105” is not limited to enclosing the entire outer peripheral surface of the local region 105. In FIG. 8, a bulk region is a region other than the local region 105 in the second metal oxide layer 204b. The oxygen deficiency in the local region 105 is larger than the oxygen deficiency in the bulk region. The oxygen deficiency of the first metal oxide layer 204a is larger than the oxygen deficiency of the bulk region. In FIG. 8, the local region 105 is in contact with the second electrode 106, penetrates through the second metal oxide layer 204 b, and is not in contact with the first electrode 103.

  In FIG. 8, the insulating film 107 has an opening 107a. In the opening 107 a, a part of the upper surface of the second electrode 106 is exposed from the insulating film 107. The exposed surface of the second electrode 106 can be in contact with gas.

  When a gas containing hydrogen atoms contacts the second electrode 106, the resistance value of the local region 105 decreases, the resistance value of the resistance film 204 decreases, and the resistance value of the gas detection element 200 decreases.

  In FIG. 9, each of the plurality of current measuring devices 930 is connected in series with a corresponding one of the plurality of gas detection elements 200. Accordingly, the plurality of current measuring devices 930 simultaneously monitor the current values flowing through the plurality of gas detection elements 200, thereby simultaneously monitoring the resistance values of the plurality of gas detection elements 200. In the present disclosure, “monitoring a resistance value” means continuously or intermittently measuring resistance value information by continuously or intermittently measuring a physical quantity (eg, current value or voltage value) related to the resistance value. Mean to get. In the present disclosure, “monitoring a plurality of resistance values at the same time” means that the periods for monitoring each resistance value partially or entirely overlap each other. For example, “simultaneously monitoring a plurality of resistance values” includes an operation in which a sequence of sequentially acquiring information on resistance values from a plurality of gas detection elements 200 is repeated for a predetermined period.

  The current measuring device 930 includes, for example, a capacitor connected in series or in parallel to the gas detection element 200, a switch connected between the gas detection element 200 and the capacitor, and a timer. For example, while the switch is on, a current flows from the gas detection element 200 to the capacitor, and charge is accumulated in the capacitor. Thereafter, when the accumulated electric charge is discharged from the capacitor, the voltage applied to the capacitor is lowered. The timer measures the time from when the charge of the capacitor is released until the voltage of the capacitor falls below a certain level. Here, the speed at which the voltage decreases reflects the resistance value of the gas detection element 200 through the amount of charge accumulated in the capacitor and the amount of current flowing from the gas detection element 200. Therefore, the time measured by the timer has resistance value information. The timer may be a counter, for example. Alternatively, the current measuring device 930 may be an ammeter. Alternatively, a voltmeter may be connected in parallel to both ends of each detection element, and resistance value information may be obtained by voltage measurement.

  The determination circuit 1014 determines whether the measurement circuit 1011 has detected a gas containing hydrogen atoms based on at least one change among the plurality of resistance values of the plurality of gas detection elements 200. For example, when the measurement circuit 1011 has N (N is an integer greater than or equal to 2) gas detection elements 200, the determination circuit 1014 has M (N is an integer greater than or equal to 2) gas detection elements 200 (M Is an integer less than or equal to 1 and less than N), it is determined that a hydrogen-containing gas has been detected. For example, M may be an integer of 2 or more. For example, the determination circuit 1014 may acquire a measurement value from the current measuring device 930 and compare the acquired measurement value with a predetermined threshold value. The determination circuit 1014 includes, for example, a semiconductor device, a semiconductor integrated circuit (IC), an LSI (large scale integration), or an electronic circuit in which they are combined. The LSI or IC may be integrated on one chip, or a plurality of chips may be combined. The LSI and IC can be called, for example, a system LSI, a VLSI (very large scale integration), or a ULSI (ultra large scale integration) depending on the degree of integration. The determination circuit 1014 may include a comparator that compares the measured value with a fixed value (for example, a threshold value). The determination circuit 1014 is, for example, a microcomputer. The determination circuit 1014 and at least a part of the measurement circuit 1011 may be integrated on one chip.

(Second Embodiment)
FIG. 11 is a circuit diagram showing an example of a gas detection circuit 1020 including the gas sensor 1000 according to the second embodiment.

  The gas detection circuit 1020 includes a measurement circuit 1021 in which a parallel circuit (that is, the gas sensor 1000) formed by connecting a plurality of gas detection elements 200 in parallel and a current measuring device 930 are connected in series, and a detection power source 920. And a power supply circuit 1012 having the same. The current measuring device 930 measures the current flowing through the gas sensor 1000 when a detection voltage is applied between the first electrode and the second electrode of the plurality of gas detection elements 200 by the detection power source 920.

  In the gas detection circuit 1020, the current measured by the current measuring device 930 increases by an amount corresponding to a decrease in the resistance value in a predetermined number of gas detection elements 200 among the plurality of gas detection elements 200, thereby generating hydrogen-containing gas. To detect. The predetermined number may be one or plural.

  More specifically, the second electrodes 106 of the plurality of gas detection elements 200 are connected to each other via the via 108 and the wiring 109 shown in FIG. 8 and connected to the positive potential terminal of the detection power source 920. Further, the first electrodes 103 of the plurality of gas detection elements 200 are connected to each other via, for example, wiring (not shown) or the like, and are connected to the current measuring device 930 at one end. The other end of the current measuring device 930 is connected to the negative potential terminal of the detection power source 920. With the above configuration, a detection voltage is applied by the detection power source 920 between the first electrode 103 and the second electrode 106 of each gas detection element 200.

  In the gas detection circuit 1020, the time point when the current value of the current measuring device 930 exceeds the predetermined threshold current shown in FIG. At the timing when the current flowing through the first one of the plurality of gas detection elements 200 exceeds the predetermined threshold current shown in FIG. 6, the current flowing through the gas sensor 1000 increases. As a result, the current value detected by the current measuring device 930 increases at this timing. Therefore, the gas detection circuit 1020 determines that hydrogen has been detected when the first one of the plurality of gas detection elements 200 exceeds a predetermined threshold current.

  As described above, as with the gas detection circuit 1010, stable hydrogen detection can be performed by the gas detection circuit 1020 using the gas sensor 1000 including the plurality of gas detection elements 200 having substantially the same structure and the same size. It becomes possible to do. In the present embodiment, the experimental results in the case of hydrogen gas have been described. However, the present invention is not limited to hydrogen gas, and the same effect can be obtained with a hydrogen-containing gas (for example, ammonia gas).

[Supplement]
As shown in FIGS. 11 and 14, the gas detection circuit 1020 includes a measurement circuit 1021 including a gas sensor 1000 and one current measurement device 930, and a determination circuit 1024. The gas detection circuit 1020 is an example of the “gas detection device” in the present disclosure, and the current measuring device 930 is an example of the “measuring device” in the present disclosure.

  In FIG. 11, the gas sensor 1000 is a parallel circuit in which a plurality of gas detection elements 200 are connected in parallel, and the current measuring device 930 is connected in series to this parallel circuit. Thereby, the current measuring device 930 can monitor the combined resistance value of the plurality of gas detection elements 200 by monitoring the combined current value flowing through the parallel circuit. In the present disclosure, “monitoring a plurality of resistance values” is not limited to individually monitoring a plurality of resistance values, and may include collectively monitoring a plurality of resistance values.

  The determination circuit 1024 captures a change in at least a part of the plurality of resistance values of the plurality of gas detection elements 200 through the change in the combined resistance value of the plurality of gas detection elements 200. Based on this change, the determination circuit 1024 determines whether or not the measurement circuit 1021 detects a gas containing hydrogen atoms. For example, the determination circuit 1024 may acquire a measurement value from the current measuring device 930 and compare the acquired measurement value with a plurality of threshold values.

(Third embodiment)
FIG. 12 is a circuit diagram illustrating an example of a gas detection circuit according to the third embodiment.

  A gas detection circuit 1030 illustrated in FIG. 12 is configured by adding a switch 950 and a reset power source 940 to the power supply circuit 1022 as compared with the gas detection circuit 1010 illustrated in FIG. 9.

  In the gas detection circuit 1030, after detecting the hydrogen-containing gas using the gas sensor 1000 and the current measuring device 930, the switch 950 is connected to the reset power source 940. By applying a reset voltage to the gas detection element 200 by the reset power source 940, the gas detection element 200 that has been in the low resistance state by the hydrogen-containing gas is electrically returned to the high resistance state.

  Thereby, it becomes possible to repeatedly detect the hydrogen-containing gas by resetting each gas detection element 200 in the low-resistance state after detection of the hydrogen-containing gas to the high-resistance state.

  In this embodiment, as an example, the reset voltage is 1.5V. The reset voltage needs to be higher than a detection voltage (eg, 0.6 V) that is always applied when monitoring the hydrogen-containing gas.

  FIG. 13 is a circuit diagram illustrating another example of the gas detection circuit according to the third embodiment.

  The gas detection circuit 1040 shown in FIG. 13 is configured by adding a switch 950 and a reset power supply 940 to the power supply circuit 1022 as compared to the gas detection circuit 1020 shown in FIG.

  Similarly to the gas detection circuit 1020, the gas detection circuit 1040 can repeatedly detect the hydrogen-containing gas by resetting the gas detection element 200 that has entered the low-resistance state after detection of the hydrogen-containing gas to the high-resistance state. It becomes.

[Supplement]
In FIG. 12, the detection power source 920 is an example of a “first power circuit” in the present disclosure, and the reset power source 940 is an example of a “second power circuit” in the present disclosure. The “power supply circuit” in the present disclosure may be, for example, the power supply itself or a conversion circuit that converts the voltage of the external power supply into a desired voltage.

(Outline of the embodiment)
A gas sensor according to one aspect includes a plurality of gas detection elements, wherein each of the plurality of gas detection elements includes a first electrode and a second electrode arranged such that main surfaces thereof are opposed to each other, and the first electrode A metal oxide layer disposed in contact with the main surface of the first electrode and the main surface of the second electrode, disposed in contact with the second electrode inside the metal oxide layer, and A local region having a greater degree of oxygen deficiency than the metal oxide layer, and an insulating film covering the first electrode, the second electrode, and the metal oxide layer, and the second electrode At least a part of the other surface facing the main surface is exposed without being covered with the insulating film, and each of the plurality of gas detection elements has the same structure and the substantially same shape. When the electrode is in contact with a gas containing gas molecules having hydrogen atoms, the first electrode and the second electrode A resistance value between the first electrode and the second electrode decreases in a predetermined number of gas detection elements among the plurality of gas detection elements. Thus, the gas is detected.

  According to such a configuration, the current flowing between the first electrode and the second electrode is concentrated in a local region where the degree of oxygen deficiency is large. As a result, the temperature of the local region can be increased with a small current.

  The local region generates heat due to the current flowing between the first electrode and the second electrode, so that hydrogen atoms are dissociated from the hydrogen molecules in the portion of the second electrode in contact with the local region. Then, the dissociated hydrogen atoms are combined with oxygen atoms in the local region of the metal oxide layer, so that a resistance value between the first electrode and the second electrode is lowered.

  More specifically, when the temperature of the local region increases, the temperature of the surface of the second electrode also increases. As the temperature rises, the efficiency of dissociating hydrogen atoms from hydrogen molecules at the second electrode is improved by the catalytic action of the second electrode.

When hydrogen molecules that have passed through the insulating film come into contact with the second electrode, hydrogen atoms are dissociated from the hydrogen molecules, and the dissociated hydrogen atoms diffuse through the second electrode and reach the local region. Then, combined with that the water (H 2 _O), oxygen deficiency of the local region is further increased and the oxygen of the metal oxide present in the local region. As a result, current easily flows in the local region, and the resistance value between the first electrode and the second electrode decreases.

  This makes it possible to detect hydrogen-containing gas without heating with a heater by utilizing self-heating and gas sensitivity in the local region formed inside the metal oxide layer, and a gas sensor with excellent power saving Is obtained.

  The reaction time during which the resistance value between the first electrode and the second electrode decreases according to the hydrogen-containing gas varies for each gas detection element and for each hydrogen-containing gas detection operation according to a certain distribution. Therefore, in the gas sensor, the hydrogen-containing gas is detected when the resistance value decreases in a predetermined number of gas detection elements among the plurality of gas detection elements. That is, the hydrogen-containing gas is detected by a predetermined number of gas detection elements whose resistance value has decreased earlier in the distribution (for example, by the gas detection elements whose resistance value has decreased earlier). Thereby, the gas sensor which can detect a hydrogen containing gas stably with sufficient sensitivity using the dispersion | variation in the said reaction time is obtained.

  In each of the plurality of gas detection elements, the metal oxide layer includes a first metal oxide layer made of the first metal oxide and an oxygen deficiency compared to the first metal oxide. And a second metal oxide layer composed of a second metal oxide having a small degree, and the first metal oxide layer is in contact with the first electrode, and the second metal oxide layer is in contact with the first metal oxide layer. The physical layer is in contact with the second electrode, and the local region is formed in contact with the second electrode through at least the second metal oxide layer, and the second metal oxide The degree of oxygen deficiency may be greater than that of the layer.

  According to such a structure, the gas sensor excellent in the detection characteristic of hydrogen containing gas is obtained by employ | adopting the laminated structure excellent in the resistance change characteristic for the said metal oxide layer.

  In each of the plurality of gas detection elements, the second electrode may be made of a material having a catalytic action for dissociating the hydrogen atoms from the gas molecules.

  According to such a configuration, hydrogen atoms are dissociated from the hydrogen molecules in the portion of the second electrode in contact with the local region, and the dissociated hydrogen atoms are in the local region of the metal oxide layer. By combining with oxygen atoms, a resistance value between the first electrode and the second electrode is lowered.

  In each of the plurality of gas detection elements, the second electrode may be made of platinum, palladium, or iridium, or an alloy containing at least one of platinum, palladium, and iridium. .

  According to such a configuration, the second electrode can dissociate hydrogen atoms from the hydrogen molecules by the catalytic action of platinum or palladium.

  The gas sensor has a plurality of series circuits formed by connecting each different one of the plurality of gas detection elements and a current measuring device in series, and the plurality of series circuits are connected in parallel. A measuring circuit, and the current measuring device is connected to the current measuring device when a detection voltage is applied between the first electrode and the second electrode of the plurality of gas detecting elements. You may measure the electric current which flows into a gas detection element.

  According to such a configuration, the hydrogen-containing gas can be detected when the current measured by the current measuring device included in the series circuit increases.

  Further, the gas sensor includes a measurement circuit in which a parallel circuit formed by connecting the plurality of gas detection elements in parallel and a current measuring device are connected in series, and the current measuring device includes the plurality of gas detection devices. You may measure the electric current which flows into the said parallel circuit, when a detection voltage is applied between the said 1st electrode and said 2nd electrode of an element.

  According to such a configuration, the current measured by the current measuring device is increased by an amount corresponding to a decrease in resistance value in a predetermined number of gas detection elements among the plurality of gas detection elements. A hydrogen-containing gas can be detected.

  Further, in each of the plurality of gas detection elements, the metal oxide layer is brought into a high resistance state and a high resistance state based on a voltage applied between the first electrode and the second electrode. It may reversibly transition to a low resistance state having a lower resistance value.

  According to such a configuration, the resistance state of the metal oxide layer can be electrically transitioned separately from the transition due to the hydrogen-containing gas. For example, the metal oxide layer may be electrically set to a high resistance state, and then a gas to be inspected may be brought into contact with the metal oxide layer, whereby a decrease in resistance value can be clearly detected. As a result, the detection characteristics of the hydrogen-containing gas are improved.

  Each of the plurality of gas detection elements may be set in a high resistance state by applying a reset voltage between the first electrode and the second electrode before detecting the gas. .

  According to such a configuration, since a decrease in resistance value in the gas detection element that is electrically set to a high resistance state is detected, the decrease in resistance value can be clearly detected, and detection of a hydrogen-containing gas is detected. Improved characteristics.

  Each of the plurality of gas detection elements may be set in a high resistance state by applying the reset voltage between the first electrode and the second electrode after detecting the gas. .

  According to such a configuration, even when the gas detection element is held in the low resistance state after detecting the hydrogen-containing gas, the hydrogen-containing gas is detected again by being electrically reset to the high resistance state. It becomes possible to do.

  In addition, the gas sensor generates a detection power source that generates a detection voltage for measuring a current flowing through the plurality of gas detection elements, and a reset voltage for setting the plurality of gas detection elements to a high resistance state. By switching between a reset power source, the detection power source, and the reset power source, one of the detection voltage and the reset voltage is provided between the first electrode and the second electrode of the plurality of gas detection elements. A power supply circuit having a changeover switch for selectively applying the power supply may be provided.

  According to such a configuration, a highly convenient gas sensor can be obtained as a module component including each power source for current measurement and high resistance (reset).

  The absolute value of the detection voltage may be smaller than the absolute value of the reset voltage.

  According to such a configuration, a gas sensor excellent in power saving can be obtained by applying a minimum voltage suitable for current measurement and high resistance (reset) to the gas detection element.

  The power supply circuit activates the characteristic that the resistance value between the first electrode and the second electrode decreases when the second electrode comes into contact with a gas containing a gas molecule having a hydrogen atom. A voltage to be applied may be constantly applied between the first electrode and the second electrode.

  According to such a configuration, it is possible to continue monitoring the leakage of the hydrogen-containing gas with a small amount of electric power by utilizing the power saving property of the gas sensor.

  In each of the plurality of gas detection elements, each of the metal oxide layers may be a transition metal oxide or an aluminum oxide.

  According to such a configuration, by using a transition metal oxide or aluminum oxide having excellent resistance change characteristics for each of the first metal oxide and the second metal oxide, A gas sensor having excellent detection characteristics can be obtained.

  In each of the plurality of gas detection elements, the transition metal oxide may be any one of tantalum oxide, hafnium oxide, and zirconium oxide.

  According to such a configuration, by using tantalum oxide, hafnium oxide, and zirconium oxide having excellent resistance change characteristics as the transition metal oxide, a gas sensor having excellent detection characteristics of a hydrogen-containing gas can be obtained. It is done.

  Further, in each of the plurality of gas detection elements, the local region generates heat by a current flowing between the first electrode and the second electrode, so that the local region of the second electrode Hydrogen atoms are dissociated from the gas molecules at the contact portion, and the dissociated hydrogen atoms are combined with oxygen atoms in the local region of the metal oxide layer, whereby the first electrode and the second electrode The resistance value between the electrodes may decrease.

  According to such a configuration, the current flowing between the first electrode and the second electrode is concentrated in a local region where the degree of oxygen deficiency is large. As a result, the temperature of the local region can be increased with a small current.

  The local region generates heat due to the current flowing between the first electrode and the second electrode, so that hydrogen atoms are dissociated from the hydrogen molecules in the portion of the second electrode in contact with the local region. Then, the dissociated hydrogen atoms are combined with oxygen atoms in the local region of the metal oxide layer, so that a resistance value between the first electrode and the second electrode is lowered.

  More specifically, when the temperature of the local region increases, the temperature of the surface of the second electrode also increases. As the temperature rises, the efficiency of dissociating hydrogen atoms from gas molecules having hydrogen atoms at the second electrode is improved by the catalytic action of the second electrode.

  When the gas molecules come into contact with the second electrode, hydrogen atoms are dissociated from the hydrogen molecules, and the dissociated hydrogen atoms diffuse through the second electrode and reach the local region. And the oxygen deficiency degree of the said local area | region increases further by couple | bonding with the oxygen of the metal oxide which exists in the said local area | region, and becoming water. As a result, current easily flows in the local region, and the resistance value between the first electrode and the second electrode decreases.

  This makes it possible to detect hydrogen-containing gas without heating with a heater by utilizing self-heating and gas sensitivity in the local region formed inside the metal oxide layer, and a gas sensor with excellent power saving Is obtained.

  A hydrogen detection method according to an aspect is a hydrogen detection method using a gas sensor including a plurality of gas detection elements, wherein each of the plurality of gas detection elements is a first surface in which main surfaces are arranged to face each other. An electrode, a second electrode, and a metal oxide layer disposed in contact with the main surface of the first electrode and the main surface of the second electrode, and having the same structure and substantially the same shape A gas containing gas molecules having hydrogen atoms is in contact with the second electrode of the plurality of gas detection elements, and the predetermined number of gas detection elements among the plurality of gas detection elements The gas is detected by a decrease in the resistance value between the first electrode and the second electrode.

  According to such a method, the gas sensor that generates heat only by the current for detecting the resistance state and detects the hydrogen-containing gas without being heated by a separate heater can detect hydrogen with excellent power saving. become.

  The gas detection device according to the present disclosure is useful in, for example, a fuel cell vehicle, a hydrogen station, a hydrogen plant, and the like.

100, 200 Gas detection element 101 Substrate 102 Insulating film 103 First electrode 104, 204 Resistance film 105 Local region 106 Second electrode 107 Insulating film 107a Opening 107b Via hole 108 Via 108 'Conductive film 109 Wiring 204a First metal oxide Material layer 204b Second metal oxide layer 300 Mask 900 Evaluation system 910 Airtight container 911 Hydrogen cylinder 912 Nitrogen cylinder 913, 914 Inlet valve 915 Exhaust valve 920 Detection power source 930 Current measuring device 940 Reset power source 950 Switch 1000 Gas sensor 1010, 1020 1030, 1040 Gas detection circuit 1011, 1021 Measurement circuit 1012, 1022 Power supply circuit 1013 Series circuit 1014, 1024 Determination circuit

Claims (27)

A measurement circuit comprising: a gas sensor including a plurality of detection cells covered with an insulating film; and at least one measuring device for monitoring a plurality of resistance values of the plurality of detection cells;
A determination circuit that determines whether or not a gas containing hydrogen atoms is detected based on at least one change among the plurality of resistance values;
Each of the plurality of detection cells is
A first electrode;
A second electrode having an exposed surface exposed from the insulating film;
A metal oxide layer disposed between the first electrode and the second electrode and including a bulk region and a local region surrounded by the bulk region and having a larger oxygen deficiency than the bulk region And
The resistance value of the detection cell decreases when the gas contacts the second electrode.
Gas detection device. The plurality of detection cells have substantially the same size and substantially the same structure,
The gas detection device according to claim 1. The plurality of detection cells are N detection cells (N is an integer of 2 or more),
The determination circuit determines that the gas is detected when the resistance value of M (M is an integer of 1 or more and less than N) of the N detection cells decreases.
The gas detection device according to claim 1 or 2. Said M is an integer greater than or equal to 2,
The gas detection device according to claim 3. The at least one measuring instrument is a plurality of measuring instruments;
The plurality of measuring devices simultaneously monitor the resistance values of the plurality of detection cells;
The gas detection device according to any one of claims 1 to 4. The plurality of measuring devices are a plurality of current measuring devices,
Each of the plurality of current measuring devices is connected in series to one corresponding detection cell among the plurality of detection cells, and monitors a current value flowing through the corresponding one detection cell.
The gas detection device according to claim 5. A power supply circuit that applies a voltage to the measurement circuit and causes a current to flow through each of the plurality of detection cells;
The gas detection device according to claim 6. The at least one measuring instrument is one measuring instrument;
The one measuring device monitors a combined resistance value of the plurality of detection cells.
The gas detection device according to any one of claims 1 to 4. The plurality of detection cells are connected in parallel.
The gas detection device according to claim 8. The one measuring device is one current measuring device;
The one current measuring device monitors a combined current value of currents flowing through the plurality of detection cells.
The gas detection device according to claim 8 or 9. A power supply circuit that applies a voltage to the measurement circuit and causes a current to flow through each of the plurality of detection cells;
The gas detection device according to claim 10. The exposed surface is configured to be in contact with the gas.
The gas detection device according to any one of claims 1 to 11. In each of the plurality of detection cells, the metal oxide layer is in contact with the first electrode, is in contact with the first metal oxide layer having an oxygen deficiency greater than the bulk region, and the second electrode, And a second metal oxide layer including the bulk region,
In each of the plurality of detection cells, the local region is in contact with the second electrode and penetrates the second metal oxide layer.
The gas detection device according to any one of claims 1 to 12. In each of the plurality of detection cells, the second electrode dissociates the hydrogen atoms from molecules contained in the gas.
The gas detection device according to any one of claims 1 to 13. In each of the plurality of detection cells, the second electrode contains at least one selected from the group consisting of platinum, palladium, and iridium.
The gas detection device according to any one of claims 1 to 14. In each of the plurality of detection cells, the detection cell is reversibly between a high resistance state and a low resistance state based on a voltage applied between the first electrode and the second electrode. Transition
The resistance value of the detection cell in the high resistance state is higher than the resistance value of the detection cell in the low resistance state.
The gas detection device according to any one of claims 1 to 4. A power supply circuit configured to apply a voltage to the measurement circuit and set the plurality of detection cells to the high resistance state before the at least one measuring instrument monitors the plurality of resistance values of the plurality of detection cells; In addition,
The gas detection device according to claim 16. A power supply circuit configured to apply a voltage to the measurement circuit and set the plurality of detection cells to the high resistance state after determining that the determination circuit has detected the gas;
The gas detection device according to claim 16. A first power supply circuit that applies a first voltage to the measurement circuit and causes a current to flow through each of the plurality of detection cells;
A second power supply circuit that applies a second voltage to the measurement circuit to set the plurality of detection cells to the high resistance state;
A switch for switching which of the first power supply circuit and the second power supply circuit the measurement circuit is connected to,
The gas detection device according to claim 6, 10 or 16. The absolute value of the second voltage is greater than the absolute value of the first voltage;
The gas detection device according to claim 19. In each of the plurality of detection cells, the metal oxide layer contains at least one of a transition metal oxide and an aluminum oxide.
The gas detection device according to any one of claims 1 to 20. The transition metal oxide is tantalum oxide, hafnium oxide, or zirconium oxide.
The gas detection device according to claim 21. The power supply circuit generates heat in the local region by applying the voltage;
The gas detection device according to claim 7 or 11. A gas detection method using a gas sensor,
The gas sensor includes a plurality of detection cells covered with an insulating film,
Each of the plurality of detection cells is
A first electrode;
A second electrode having an exposed surface exposed from the insulating film;
A metal oxide layer disposed between the first electrode and the second electrode and including a bulk region and a local region surrounded by the bulk region and having a larger oxygen deficiency than the bulk region And
The resistance value of the detection cell has a characteristic of decreasing when the gas contacts the second electrode,
The gas detection method includes:
Monitoring a plurality of resistance values of the plurality of detection cells;
Determining whether a gas containing hydrogen atoms is detected based on a change in at least one of the plurality of resistance values.
Gas detection method. The plurality of detection cells are N detection cells (N is an integer of 2 or more),
In the determining step, when the resistance value of M (M is an integer of 1 or more and less than N) of the N detection cells decreases, it is determined that the gas is detected.
The gas detection method according to claim 24. In the monitoring step, the resistance values of the plurality of detection cells are simultaneously monitored.
The gas detection method according to claim 24 or 25. Monitoring the combined resistance value of the plurality of detection cells in the monitoring step;
The gas detection method according to claim 24 or 25.

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